.NH 2
Pointers and subroutine calls
.PP
The theory outlined above assumes that variables can
only be changed by a direct assignment.
This condition does not hold for EM.
In case of an assignment through a pointer variable,
it is in general impossible to see which variable is affected
by the assignment.
Similar problems occur in the presence of procedure calls.
Therefore we distinguish two kinds of definitions:
.IP -
an \fIexplicit\fR definition is a direct assignment to one
specific variable
.IP -
an \fIimplicit\fR definition is the potential alteration of
a variable as a result of a procedure call or an indirect assignment.
.LP
An indirect assignment causes implicit definitions to
all variables that may be accessed indirectly, i.e. 
all local variables for which no register message was generated
and all global variables.
If a procedure contains an indirect assignment it may change the
same set of variables, else it may change some global variables directly.
The KILL, GEN, IN and OUT sets contain explicit as well
as implicit definitions.